The present invention relates generally to data transmission in communication systems and more specifically to methods and systems for uplink (UL) and downlink (DL) coordinated multi-point transmission (CoMP) to reduce radio link failure (RLF) during an inter-base station or cell handover.
As used herein, the terms “user agent” and “UA” can refer to wireless devices such as mobile telephones, personal digital assistants (PDAs), handheld or laptop computers, and similar devices or other User Equipment (“UE”) that have telecommunications capabilities. In some embodiments, a UE may refer to a mobile, wireless device. The term “UE” may also refer to devices that have similar capabilities but that are not generally transportable, such as desktop computers, set-top boxes, or network nodes. Generally, throughout the present disclosure the terms UE and UA are interchangeable.
In traditional wireless telecommunications systems, transmission equipment in a base station or other network node transmits signals throughout a geographical region known as a cell. As technology has evolved, more advanced equipment has been introduced that can provide services that were not possible previously. This advanced equipment might include, for example, an evolved universal terrestrial radio access network (E-UTRAN) node B (eNB) rather than a base station or other systems and devices that are more highly evolved than the equivalent equipment in a traditional wireless telecommunications system. Such advanced or next generation equipment may be referred to herein as long-term evolution (LTE) equipment. Additional improvements to LTE systems and equipment will eventually result in an LTE advanced (LTE-A) system. As used herein, the phrase “base station” or “cell” will refer to any component, such as a traditional base station or an LTE or LTE-A base station (including eNBs), that can provide a UE with access to other components in a telecommunications system.
In mobile communication systems such as the E-UTRAN, a base station provides radio access to one or more UEs. The base station comprises a packet scheduler for dynamically scheduling downlink traffic data packet transmissions and allocating uplink traffic data packet transmission resources among all the UEs communicating with the base station. The functions of the scheduler include, among others, dividing the available air interface capacity between UEs, deciding the transport channel to be used for each UE's packet data transmissions, and monitoring packet allocation and system load. The scheduler dynamically allocates resources for Physical Downlink Shared CHannel (PDSCH) and Physical Uplink Shared CHannel (PUSCH) data transmissions, and sends scheduling information to the UEs through a control channel.
To facilitate communications, a plurality of different communication channels are established between a base station and a UE including, among other channels, a Physical Downlink Control Channel (PDCCH). As the label implies, the PDCCH is a channel that allows the base station to control the UE during downlink data communications. To this end, the PDCCH is used to transmit scheduling assignment or control data packets referred to as Downlink Control Information (DCI) packets to the UE to indicate scheduling to be used by the UE to receive downlink communication traffic packets on a Physical Downlink Shared Channel (PDSCH) or transmit uplink communication traffic packets on Physical Uplink Shared Channel (PUSCH) or specific instructions to the UE (e.g. power control commands, an order to perform a random access procedure, or a semi-persistent scheduling activation or deactivation). A separate DCI packet may be transmitted by the base station to the UE for each traffic packet/sub-frame transmission.
It is generally desirable to provide a high data rate coverage using signals that have a high Signal to Interference Plus Noise ratio (SINR) for UEs serviced by a base station. Typically, only those UEs that are physically close to a base station can operate with a very high data rate. Also, to provide high data rate coverage over a large geographical area at a satisfactory SINR, a large number of base stations are generally required. As the cost of implementing such a system can be prohibitive, research is being conducted on alternative techniques to provide wide area, high data rate service.
Coordinated multi-point (CoMP) transmission and reception may be used to increase transmission data rate and/or signal quality in wireless communication networks such as LTE-A networks. Using CoMP, neighboring base stations coordinate to improve the user throughput or signal quality, especially for users at a cell edge. CoMP may be implemented using a combination of base stations such as eNBs, and/or relay nodes (RN) and/or other types of network nodes and/or cells.
FIG. 1 is an illustration of an exemplary architecture for an LTE network implementing CoMP to provide UE 10 with an improved SINR. As shown in FIG. 1, UE 10 is either located within or close to the zones of radio coverage (e.g., cells) of each of base stations 100, 102, and 104. Because UE 10 is close to each of the cells established by the base stations, UE 10 is able to receive a signal broadcast by each of the base stations. The channels are indicated by the labels H11, H21, and H31 in FIG. 1.
In LTE-A, for example, CoMP can be used to improve the throughput for cell edge UEs as well as the cell average throughput. There are two primary mechanisms in which CoMP transmissions may be implemented to recognize these improvements. First, CoMP transmissions may provide coordinated scheduling, where data is transmitted to a single UE from one of the available transmission points (e.g., one of base stations 100, 102 and 104 on FIG. 1) and scheduling decisions are coordinated to control, for example, the interference generated in a set of coordinated cells. Secondly, CoMP transmissions may provide joint processing where data is simultaneously transmitted to a single UE from multiple transmission points, for example, to (coherently or non-coherently) improve the received signal quality and/or actively cancel interference for other UEs.
In the case of coordinated scheduling, data is only transmitted by the serving cell, but the scheduling decisions are made with coordination among the neighboring cells. In the case of joint processing CoMP transmission, multiple base stations transmit the data to the same user. The UE then jointly processes the transmissions from multiple nodes to achieve a performance gain.
In an LTE network, radio link failure can happen during a handover procedure, such as that described in TS 36.300 v V8.8.0, “Evolved Universal Terrestrial Radio Access (E-UTRA) and Evolved Universal Terrestrial Radio Access Network (E-UTRAN); Overall description; Stage 2 (Rel 8)”. Radio link failure may be due to several factors, including rapid radio channel degradation (e.g., due to sudden co-channel interference, or a UE operating near sensitivity limits). If the channel conditions deteriorate so fast that the UE is unable to process or receive a handover command, the UE initiates the radio link recovery process. Currently, when a UE enters the radio link recovery state, depending on whether the target base station has been prepared, the UE has several options. For example, FIG. 2 illustrates defined phases that occur during a radio link recovery process. After the UE detects the radio link problem at point 106, the UE enters a first phase of radio link recovery by starting timer T1 in step 108. If the UE cannot recover the radio link before T1 expires, the UE considers the radio link to have failed and enters a UE-based mobility condition.
During UE-based mobility, the UE starts a second timer T2 in step 110 and attempts to access a new cell to re-establish the radio link by connecting to a new base station. The new base station is selected based on the UE's prior monitoring of network conditions (e.g., the UE will attempt to connect to the base station having the strongest radio link with the UE). If the new base station has already received the UE context from the original serving base station or cell (the base station or cell that suffered from the radio link failure), the UE may stay in an RRC_Connected state and continue handover procedure and establish the radio link with the new base station in step 112. If, however, the new base station has not previously received the UE context from the original serving base station or cell, the UE may go into an RRC_IDLE state. At that point, the UE goes on to perform a normal IDLE mode to ACTIVE mode transition to the new cell. The IDLE mode to ACTIVE mode transition may lead to a longer interruption time in the range of hundreds of milliseconds to several seconds longer than normal handover.
With the introduction of CoMP technology, the data transmission quality can be improved, especially at the cell edge. However, the current CoMP techniques only apply to data channels. As a result, the control channel is only provided by the serving base station or cell. Consequently, conventional implementations of CoMP only assist with the reduction of data channel radio link failure, but do not mitigate control channel radio link failures.